If there is STEP7 on the PG, then go find the original installation CDs for the PG. You can install all the relevant programs to another PC, however if the PG is very old, then some of the programs may be too old to run on a current Windows.
And yes, most Siemens programs require an authorisation to run.
To learn S7-300, STEP7 LITE is enough. This you can download for free from Siemens (200MB !). Search for "STEP7 LITE" on this site and you will find a link to it.
You will also need an online cable, unless you just want to study programming offline.